Actually, everybody is misreading this rule in the plan. It says that "School and municipal buses...may be used to provide transportation for individuals..." It doesn't say they must be used for that purpose.
The mayor made a bad call: the proviso makes it his or her call to use the busses. His call was not to. That's not a deviation from the plan. He 'may' choose to use buses, and he chose not to.
It was the wrong decision, but not in violation of the plan.
The captain of the Titanic may also "choose" not to use all of his lifeboats, or to lower them half full.